Introduction to Medical Imaging

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Introduction to Medical Imaging

• BME/EECS 516
• Douglas C. Noll
• (edited by JF)

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Medical Imaging

• Non-invasive visualization of internal organs,
  tissue, etc.
• Is endoscopy an imaging modality?
• Image a 2D signal f(x,y) or 3D f(x,y,z)
• Is a 1D non-imaging sensing techniques an imaging
  modality?

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Major Modalities

• Projection X-ray (Radiography)
• X-ray Computed Tomography (CT)
• Nuclear Medicine (SPECT, PET)
• Ultrasound
• Magnetic Resonance Imaging

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Projection X-ray Imaging
Object
X-ray Detector
Id(x,y)
m(x,y,z)
X-ray Source

• Image records transmission of x-rays through
  object
• The integral is a line-integral or a projection
  through obj
• m(x,y,z) x-ray attenuation coefficient, a
  tissue property, a function of electron density,
  atomic ,

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Projection X-ray Imaging
MRI
NearIR
Transmissivityof body
Diagnostic X-ray Band
EM SpectrumEnergy
Visible

• X-ray imaging requires interactions of x-ray
  photons with object work in a specific energy
  band
• Above this band body is too transparent
• Below this band body is too opaque
• Well below this band wavelengths are too long
• One problem with x-ray imaging no depth (z) info

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X-ray ImagingProjection vs Tomographic
Chest Mass
Cross-sectional Image
Projection Image
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X-ray Computed Tomography
Collimator
X-ray Source
Object
m(x,y,z0)
X-ray Detector

• Uses x-rays, but exposure is limited to a slice
  (or a couple of slices) by a collimator
• Source and detector rotate around object
  projections from many angles
• The desired image, I(x,y) m(x,y,z0), is
  computed from the projections

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X-ray Computed Tomography
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Anatomical vs Functional Imaging
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Nuclear Medicine (Scintigraphy)
Pinhole Camera
Object
Gamma Detector
Id(x,y)
GammaSource

• Detector records emission of gamma photons from
  radioisotopes introduced into the body
• The integral is a line-integral or a projection
  through obj
• Source s(x,y,z) usually represents a selective
  uptake of a radio-labeled pharmaceutical

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Nuclear Medicine (Scintigraphy)

• Issue Pinhole Size
• Large pinhole more photons, better SNR
• Large pinhole more blur, reduced resolution
• Issue Half-life
• Long half lives are easier to handle, but
  continue to irradiate patient after imaging is
  done
• Issue Functional Specificity
• Pharmaceuticals must be specific to function of
  interest
• E.g. Thallium, Technicium
• Issue No depth info
• Nuclear Medicine Computed Tomography (SPECT, PET)

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Nuclear Medicine (Scintigraphy)
Bone Scan
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SPECT Scanner (3 heads)
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Nuclear Medicine (SPECT)
Short Axis
Long Axis
Long Axis
Cardiac (Left Ventricle) Perfusion Scan
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PET Scanner
http//upload.wikimedia.org/wikibooks/en/f/fb/PetD
iag2.jpg
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PET-CT Scanner
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PET-CT Scan
Anatomy
Function
Both
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Ultrasound Imaging
Object
ImageR(x,y)
Transducer position
Transducer
R(x,y,z)
Depth

• Image reflectivity of acoustic wave, R(x,y,z).
• Depth A function of time (ping-echo)
• Lateral Focusing of wavefronts
• Direct imaging (e.g. vs. computed) modality
  echo data is placed directly into image matrix

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Ultrasound Imaging

• Issue Transmit Frequency
• Increase in frequency reduces wavelength
• Reduced (improved) resolution size (2-3 l)
• Also improved lateral resolution (diffraction)
• Increases attenuation (and thus, range of depth)
• Issue Flow
• Can use Doppler effect to image flow
• Issue Speckle
• Most noise in US is speckle (signal dependent)

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Ultrasound Imaging
High-Resolution
Color Doppler
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Magnetic Resonance Imaging

• Atomic nuclei and hydrogen nuclei, 1H, in
  particular, have a magnetic moment
• Moments tend to become aligned to applied field
• Creates magnetization, m(x,y,z) (a tissue
  property)
• MRI makes images of m(x,y,z)

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Magnetic Resonance Imaging
RF Excitation (Energy into tissue)
Magnetic fields are emitted

• The magnetization is excited into an observable
  state
• Magnetization emits energy at a resonant
  frequency

???????
(63 MHz at 1.5 T)
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Magnetic Resonance Imaging

• Frequency is proportional to magnetic field
• We can create a frequency vs. space variation
• Use Fourier analysis to determine spatial
  location
• Interestingly, l is much larger than resolution
  not imaging EM direction, but using its frequency

?(x,y,z)??????(x,y,z)
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MRI
cancer
cardiac
stroke
joint
neuro function
lung